Brushless motor having isosceles sided stator coils and position detection capability

ABSTRACT

A three-phase brushless motor includes a rotor with a permanent magnet having P (P is an integer not less than two) polarities and a stator facing the rotor and having plural coils shaped in approx, triangle or trapezoid. A space between adjacent coils is (360/P)×(5/3) degree. Three position-detectors, which detect the position of the rotor, is placed at intervals of (360/P)×(2/3) degree in an area where no coils are placed. This structure allows the coils to be optimally shaped and placed, and realizes to reduce a number of coils as well as improve the motor characteristics.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCTJP00/01925.

TECHNICAL FIELD

The present invention relates to a three-phase brushless motor, and moreparticularly it relates to a placement of stator coils of the motor.

BACKGROUND ART

A brushless motor (hereinafter referred to as a motor) has been requiredto have greater motor-constant “Kt” representing the torque generatedper unit-electric-current in order to output greater power. For gettingthe greater motor-constant “Kt”, in general, winding-coils are disposedoccupying an area equal to or more than an area occupied by permanentmagnets (hereinafter referred to as a magnet) in a disc-shaped rotor.Then a number of coils “S” increases or a number of turns of each coilincreases. In this case, however; if a stator is formed by disposingflat-coreless-coils on a printed circuit board, the motor becomesexpensive and incurs low productivity because the cost of manufacturingequipment rises and a number of processes increase.

When motors are downsized, the area to be occupied by a bundle ofwinding of the coil naturally decreases. Thus the number of turns of thecoil cannot be increased As a result, the motor constant cannot begreater. This problem provides users with the following alternative: Oneis just to persuade oneself that the downsizing of the motor entails thesmaller motor constant, the other is to change windings and magnets toones of higher performance and more expensive so that the influence, dueto downsizing the motor, decreasing the motor constant can be minimized.

FIG. 2A through FIG. 2C show a conventional facing and flat three-phasesbrushless motor. Meanwhile, the facing and flat type is referred to as amotor structure where a rotor faces a stator via a spacing in the axialdirection.

In FIG. 2B, a size of the motor is represented by diameter “OD”extending between an outer rim of a group of the coils. When diameter“OD” measures ca. 40 mm, another diameter “ID” extending between aninner rim of the group of the coils measures ca. 20 mm. In many cases,nine coils are employed by considering the balance between the radiallength and circular length of the coil. In this case, since the numberof polarities “P” of the disc-shaped rotor shown in FIG. 2C is 12, thefan-shaped polarity forms an angle of 30° based on 360/P. The coils arearranged as shown in FIG. 2B, i.e. respective coils are spaced atintervals of 360/9=40°. Thus the space between each coil is 4/3 of thewidth of the polarity. In other words, the relation (coil placementcondition) between the polarity width and the space between each coil isexpressed as (360/P)×(4/3), with considering a placement of thepolarities. Thus in this case, 40 degree interval is calculated.

In FIG. 2B, respective coils forming U, V and W phases are placed. Uphase is formed by coils U1, U2 and U3. V phase is formed by coils V1,V2 and V3. W phase is formed by coils W1, W2 and W3. Nine coils in totalare disposed on printed circuit board 2. Width “A” of winding-bundle ofeach coil is restricted by soldering land 4 and the adjacent coils. Thesoldering land 4 is disposed inside of each coil and used forterminating the coil-wire end.

Each coil, in particular, comprises numbers of winding-bundles,therefore, 0.01 mm dispersion of winding diameter causes 0.2 mmdispersion on coil's outer diameter when the winding turns 20 ties. Thisdispersion and the work of fixing each coil onto the printed circuitboard should be taken into consideration, the space to the adjacentcoils thus should be ca. 1 mm in general. Further, another space isrequired for disposing magnetic sensor 5—a position detector—fordetecting a rotor position. Sensors 5 are placed inside respective threecoils, i.e. coils U1, V1 and W1. The width “A” of winding-bundle ofthese three coils thus become narrower than those of other 6 coils. Thisrestriction reduces a number of turns of these three coils, and preventsthe motor constant from becoming greater. The size of the magneticsensor and the area of the soldering land are difficult to be reduced inproportion to downsizng the motor, therefore, the influence of thisrestriction adversely increases more than proportional at greaterdownswing of the motor.

When an isosceles angle of the coil matches up to 30 degree which therotor polarity forms, windings of the coil outwardly bulge out by ca.1.7 mm on each side, and inwardly bulge out by ca. 0.9 mm on each side.When the space between the adjacent coil is considered, there is littlespace for disposing the winding outside the isosceles angle 30 degree.Therefore, if the number of turns of the coil should be increased,almost of all the windings should be disposed inside the angle 30degree. As a result, the isosceles angle becomes practically less than30 degree.

As such, in the case that the isosceles angle becomes smaller, one of arotor polarity reaches the position, in a winding-bundle of one ofisosceles sides, where the maximum torque is produced, then the otherrotor polarity is displaced from the position, in another winding-bundleof the other side of isosceles sides, where the maximum torque would beproduced. Therefore, as one entire coil, this coil cannot produce themaximum torque. As a result, the motor constant becomes smaller than thecase where the isosceles angle forms 30 degree.

In the case that much more coil's windings are disposed inside ofpolarity angle 30 degree, a point, where torque is produced in adirection reverse to normal rotating direction of the motor, isprovided, so that the motor constant incurs some loss. This reversalpoint also causes vibration of the motor.

FIG. 2A illustrates this situation. FIG. 2A shows a relation betweencross section of winding-bundle of the coil and a position of the rotorshown in FIG. 2B. Coil's winding-bundles 8 and 9 are disposed withrespect to polarity 7 disposed every 30 degree in rotor 6.Winding-bundles 8 and 9 form the same windings, and e.g. when electriccurrent runs from this side to that side in bundle 8, the current runsfrom that side to this side in bundle 9. In other words, when differentpolarities face bundles 8 and 9 respectively, torque is produced in thesame direction respectively; however, when the same polarities facebundles 8 and 9, torque is generated in the reverse direction andcancels each other. As shown in FIG. 2A and FIG. 2B, numbers of windingsare disposed inside the 30 degree, and in this case, when rotor 6revolves and polarity 7 reaches the position shown in FIG. 2A, Z-sectionof bundle 9 faces the same polarity of bundle 8. Thus reverse torque isproduced in the same bundle 9.

The following prior art has been known for addressing the problemdiscussed above: Sensors 5 are collected and placed in the area wherethe coils are not disposed, as shown in FIG. 3. The coils are placed atconventional intervals of (360/P)×(4/3). For instance, 10 polaritieswith 6 coils can improve the problematic situation. In FIG. 3, based onthe conventional coil placement condition discussed above, respectiveoils forming U, V and W phases are placed at intervals of 48 degreebecause of P=10. In other words, U phase coils are formed by coils U1and U2, V phase coils are formed by coils V1 and V2, and W phase coilsare formed by coils W1 and W2. In total 6 coils are disposed on theprinted circuit board.

These 6 coils are massed in five areas disposed in every 48 degree, i.e.total area covered by 48×5=240 degree, so that the space foraccommodating sensors 5 can be obtained. Sensors 5 can be placed atintervals of (360/P)×(4/ 3) degree or (360/P)×(2/3) degree. The exampleshown in FIG. 3 shows the interval of (360/P)×(2/3) degree i.e. 24degree.

However, when diameter “OD”, extending the outer rim formed by eachcoil, is small, soldering lands 3 outside the respective coil-wire endsbecome closer to sensor 5. Thus when the end of each coil-wire issoldered, the coil-wire end tends to short with the terminal of sensor5. The coil-wire end is, in particular, easy to be deformed so that itis difficult to regulate its position during the work, and the disperseof length of coil-wire end is large due to terminating work, thus thecoil-wire ends need, in general, a rather longer length preliminarily.Soldering lands 3 are placed on printed circuit board 2; however, themanufacturing technique allows rather great disperse on positionalaccuracy of lands 3, so that ca. 0.2 mm relative displacement withsensor 5 should be allowed. Due to the reasons discussed above, enoughspace between land 3 and sensor 5 is needed Therefore, when diameter“OD” is small, sensors 5 cannot be massed outside the coils, and theyshould be placed inside the coils instead.

In the case of the conventional motor discussed above and whose diameter“OD” is not more than 40 mm, spaces for respective coils should besmall. Further, the number of turns of the coils are difficult toincrease because the space for the soldering lands and the magneticsensors should be reserved as well as good workability should beprepared. As a result, the motor constant cannot be greater. On thecontrary, the motor constant decreases more than proportional at greaterdownsizing of the motor when the area ratio is considered between coils'occupying area vs. the area occupied by the magnetic sensors besidesconsidering the dimensionally restricted soldering lands.

Other problems have been already discussed: i.e. the isosceles angle ofthe coil is practically smaller than the fan-shaped isosceles angle thatforms the rotor polarity, so that the torque generated incurs some loss,and reverse torque is produced to counteract the torque and causevibration of the motor.

DISCLOSURE OF THE INVENTION

The present invention addresses the problems discussed above, and abrushless motor of the present invention comprises the followingelements:

a rotor with a permanent magnet having “P” (P is an integer equal to twoor more than two) pieces of polarity; and

a stator facing the rotor and having a plurality of coils. The coil hasisosceles sides which interlink with the magnetic field produced by thepolarities, and extension lines of the isosceles sides—extending throughcenters of winding-bundles of the coil—toward a shaft center cross eachother at the shaft center and form an angle of 360/P degree.

This structure restrains the lowering of a motor constant due todownsizing the motor. Further, an optimum shape and optimum placement ofthe coils allow a number of coils to be reduced from the same size ofthe motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross section illustrating the relation between rotor'sposition and winding-bundles of a brushless motor in accordance with anexemplary embodiment of the present invention.

FIG. 1B shows a stator of the brushless motor in accordance with theexemplary embodiment of the present invention.

FIG. 1C shows a rotor of the brushless motor in accordance with theexemplary embodiment of the present invention.

FIG. 2A is a cross section illustrating the relation between rotor'sposition and winding bundles of a conventional brushless motor (12polarities and 9 oils).

FIG. 2B shows a stator of the conventional brushless motor.

FIG. 2C shows a rotor of the conventional brushless motor.

FIG. 3 shows a stator of another conventional brushless motor (10polarities and 6 coils).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An exemplary embodiment of the present invention is demonstratedhereinafter with reference to the accompanying drawings.

The exemplary embodiment is demonstrated with reference to FIGS. 1A, 1Band 1C.

FIG. 1A is a cross section illustrating the relation between rotor'sposition and winding-bundles of a brushless motor in accordance with theexemplary embodiment of the present invention. FIG. 1B shows a stator ofthe same brushless motor, and FIG. 1C shows a rotor of the samebrushless motor.

FIG. 1A through FIG. 1C illustrate that the present invention reduces 9coils conventionally required to 6 coils which are employed in athree-phase brushless motor having 12 rotor-polarities (P) and thediameter measures ca. 40 mm. Each coil is formed by flat corelesswindings and preferably shaped in roughly a triangle or a trapezoid. Therotor is shaped in a disc.

As shown in FIG. 1B, respective coils forming U-phase, V-phase andW-phase are placed at intervals of (360/P)×(5/3) degree. Since P=12, theinterval is calculated to be 50 degree. In other words, U-phase isformed by coils U1 and U2, V-phase is formed by coils V1 and V2, andW-phase is formed by coils W1 and W2. As such, three phases of thestator are formed by two coils of respective phases. The above formulathus results in 50 degree interval.

As shown in FIG. 1B, the coil winding-bundle forming isosceles sides ofcoil V1 is placed with respect to the one of angle sides forming 360/Pdegree so that one of the isosceles sides and the other side are placedwithin 360/(4×P) degree both inside and outside of the angle side. Inthe case of P=12, angle sides-extending through the center of twowinding-bundles of a coil-form 30 degree. Each winding-bundle is placedwithin 7.5 degree inside and outside the angle side as a center.

Thus, for instance, in the case of the motor having diameter OD of ca.40 mm, diameter ID is, in general, ca. 20 mm due to structuralrestriction by bearings and the like. In this case, when the isoscelessides of each coil form 30 degree, the windings piled up from the bundlecenter measures 3.5 mm width on both sides at the outer rim (φ40) and1.7 mm width on both sides at the inner rim (φ20). This structure allowsthe windings to be placed outside the angle of 30 degree with enoughspace even if the clearance between the adjacent coil is reserved.

When six coils are arranged at intervals of 50 degree, the area where nocoils are placed extends over 60 degree (in this embodiment, the areaextends 65 degree.) In this area, three magnetic sensors HU, HV, HW,which are position detectors of the rotor, are placed at intervals of(360/P)×(2/3) i.e. 20 degree. This structure allows the place forrespective coils to be larger than the case where the magnetic sensorsare placed within the coils.

Relations of physical placement between the three phase coils and threeposition detectors are further detailed hereinafter with reference toFIG. 1B. The stator coil comprises three phases, namely, U-phase,V-phase and W-phase. Each phase is formed by coils U1 and U2 coupled inseries, V1 and V2 coupled in series, W1 and W2 coupled in series. Thecenter of coil U1 is spaced out from position detector HU by 60 degree.In the same manner, the center of coil V1 is way from position detectorHV by 90 degree, and the center of coil W1 is away from detector HW by180 degree. Respective detectors are arranged at intervals of 20 degree.

The structure of the three-phase motor in accordance with the embodimentpractically makes the three-phase motor operative as same as theconventional brushless motor. This is demonstrated hereinafter.

In the conventional motor, with respect to a signal detected in aU-phase oil, a signal detected in a V-phase coil is shifted by 240degree in electrical angle. With respect to the signal detected in theV-phase coil, a signal detected in a W-phase coil is shifted by also 240degree in electrical angle. Thus the signal in the W-phase coil isshifted from the signal in the U-phase coil by 480 electrical angle;however a shift by 360 electrical angle is identical to the same phase,thus the signal in the W-phase coil is eventually shifted from that inthe U-phase coil by 120 electrical angle. The detected signals in thethree phases are shifted by 120 electrical angle respectively. This isthe condition for the three-phase brushless motor to function.

On the other hand, in the brushless motor of the present invention, withrespect to a signal detected in coil U1, a signal detected in coil V1 isshifted by 300 degree in electrical angle. With respect to the signaldetected in coil V1, a signal detected in coil W1 is shifted by 300electrical angle. As a result, the signal detected in coil W1 is shiftedfrom that in coil U1 by 600 electrical angle. Since the shift of 360electrical angle turns out the same phase, the signal in coil W1 iseventually shifted from the signal in coil U1 by 240 electrical angle.If coil W1 is shifted from coil U1 by 480 degree as same as theconventional motor, the three-phase brushless motor of the presentinvention satisfies the condition for functioning as the motor; however,the shift value is 300 degree. This does not allow this motor tofunction as the motor. Then the winding of coil V1 is wound in theopposite direction to that of coil U1, thereby shifting coil V1 fromcoil U1 by 180 electrical angle. As a result, coil V1 is shifted fromcoil U1 by 480 degree.

With respect to coil U1, coil U2 is shifted by 150 degree in mechanicalangle, that means the shift amount is 900 degree in electrical angle.Since 360 degree shift in electrical angle is equivalent to the samephase, coil U1 has 180 degree phase difference in electrical angle fromcoil U2. Therefore, the winding of coil U2 is wound in the oppositedirection to that of coil U1 so that coils U1 and U2 can be in the samephase in electrical angle.

In the same manner, coils V1 and V2, coils W1 and W2 are shifted by 150degree in mechanical angle respectively. Thus the winding of one of thecoils is wound in the opposite direction to that of the other, so thatboth the coils in respective cases are in the same phase in electricalangle. In other words, three windings of coils U2, V1 and W2 are woundin the opposite directions to those of coils U1, V2 and W1, so that thesignals of respective coils of three phases are shifted by 120electrical angle from each other. As a result, this motor can functionas a three-phase brushless motor.

The effect discussed above can be realized also by changing a couplingdirection between respective coils through wiring the printed circuitboard, instead of changing the winding direction of the coil.

As discussed above, the structure of the brushless motor of the presentinvention allows the coils to be disposed at optimal places and shapedin an optimal form, so that loss of a motor constant is restrained. Inthe attempt at downsizing a motor, the loss of the motor constant due torestriction of placing coils can be reduced, thereby increasing themotor constant. Further, a number of coils can be reduced, thauks to theoptimal placement and optimal shape of the coils, from the conventionalmotor of the same size.

Industrial Applicability

The present invention relates to coil placement in a stator applicableto a brushless motor. It be more specific, it relates to a three-phasebrushless motor comprising a rotor with a permanent magnet having Ppieces of polarity and a stator facing the rotor and coming with aplurality of coils forming approx. triangles or trapezoids. In thisthree-phase brushless motor, space between adjacent coils measures(360/P)×(5/3) degree, and three position detectors for detecting theposition of the rotor are placed at intervals of (360/P)×(2/3) degree.The detectors are placed in the area where no coils are disposed. Thisstructure allows the coils to be optimally placed and shaped in optimalform, and also realizes to reduce a number of coils as well as increasea motor constant.

What is claimed is:
 1. A brushless motor comprising: a rotor with apermanent magnet having P (P Is an integer not less than two), whereintwo extension lines, which extend toward a shaft center of said rotoralong both ends of each magnetic polarity of said rotor, form an anglewith respect to the shaft center of said rotor; and a stator facing saidrotor and having a plurality of coils, wherein any one of the coils haswinding-bundles including isosceles sides interlinking with a magneticfield generated by the magnetic polarities, wherein two extension linesextending along centers of the winding-bundles of the isosceles sides ofthe coil cross each other at the shaft center of said rotor and form avertex angle of 360/P degrees, and wherein the vertex angle formed bythe two extension lines extending along centers of the winding bundlesis equal to the angle formed by the two extension lines extending alongboth ends of the each magnetic polarity of said rotor.
 2. The brushlessmotor as defined in claim 1, wherein an outer rim of the coil measuresnot more than φ40 mm.
 3. The brushless motor as defined in claim 1,wherein the coli winding-bundles forming the isosceles sides aredisposed within an area covered by an angle of 360/(4×P) degree bothinside and outside with respect to a center of the angle of 360/Pdegree.
 4. The brushless motor as defined in claim 3, wherein the coilsadjacent to each other are spaced out at intervals of (360/P)×(5/3)degree.
 5. The brushless motor as defined in claim 4 further comprisingthree position detectors for detecting a position of said rotor, whereinsaid detectors are placed at intervals of (360/P)×(2/3) degree and in anarea where the coils are not placed.